Nieves was the Red Sox pitching coach since November 2012, overseeing a staff that was critical to Boston's 2013 World Series run but had posted a 4.86 team ERA this season, second-highest in baseball. The Red Sox entered Thursday 13–15, last in the American League East.
Boston was off Thursday night and will begin a 10-game road trip on Friday, starting with three in Toronto against the Blue Jays.
"We are currently working on a replacement," general manager Ben Cherington told reporters. "If that person's not in place by tomorrow night in Toronto, then [manager] John [Farrell] and the rest of the staff will handle the duties."
Farrell was the Red Sox pitching coach from 2007 to 2010. He mentioned a lack of stability among the rotation as an issue; Rick Porcello is the only Boston starter with a sub-5.00 ERA at 4.38. The Red Sox starters have a 5.54 ERA.
"What it comes down to is focusing on the rotation, because the rotation pitches the bulk of the innings, quality required, stability needed," Farrell said. "Whether or not that same connection was made with the current group to bring about the consistency of performance... as Ben stated, there's a number of things that go into this. But the ability to maintain the consistency or affect change when needed or [make] adjustments that are required that's typical with any pitcher, that wasn't the same as we saw... two years ago."
Before joining the Red Sox, Nieves, 50, spent 14 seasons as a coach in the White Sox organization.
- Mike Fiammetta